As humanity seeks to decarbonise hard-to-abate sectors, electrolytic hydrogen production, based on renewable electricity, is expected to grow significantly. Co-location of hydrogen and electricity infrastructure is sometimes desirable for reasons of cost and practicality and is unavoidable at the electrical grid interface of an electrolyser. In this work, the risks of co-location for GW-scale hydrogen electrolysers and their grid-connection substation are explored in an interdisciplinary manner by considering the ignition sources presented by electrical substation infrastructure along with the accident scenarios that might arise with varying degrees of co-location, given the properties of hydrogen gas. Corona discharge and insulator discharge are identified as the two greatest hazards present. Mitigation measures are discussed in relation to the accident scenarios through a synthesis of electrical design and hydrogen fire safety design, considering jet flame and explosion risks with electrical assets as ignition sources. In many cases, ignition sources and fuel leak risk cannot be eliminated but physical separation and inclusion of barriers can offer adequate mitigation. Detailed recommendations for future research are identified.